1. Field of the Invention
The present invention relates to hypodermic syringes. More particularly, the present invention relates to a syringe having structure for withdrawing the hypodermic needle into the syringe barrel after use.
2. Description of Related Information
Generally speaking, a syringe includes a cylindrical barrel commonly made of thermoplastic material or glass, having a distal end connected to a sharpened needle cannula and a proximal end adapted to receive a stopper and plunger assembly.
In recent years there has developed an increased concern regarding the transfer of disease, infection or the like to syringe users and health care professionals who accidentally, or through negligent handling, stick themselves with hypodermic needles while disposing of used hypodermic syringe products. In many areas in a hospital, where needle cannula products are used, disposal bins are provided so that a syringe or other needle cannula product may be immediately discarded in a safe rigid container. However, there are areas of medical practice, such as emergency rooms, where disposal containers are not readily available or practical, and where products having self-contained safety features are desirable. In theory, after such a syringe is used to inject medication or for another purpose, a safety device contained within the syringe is activated to prevent further contact with the sharp needle tip. One type of safety syringe includes structure which allows the withdrawal of the hypodermic needle into the syringe barrel to minimize the chance of further contact with the sharp needle tip. A syringe with its needle withdrawn can be more safely transported to a disposal system.
One such prior art retractable needle syringe includes a frangible zone which allows separation of the forward wall of the barrel, which is connected to the hypodermic needle, from the side wall of the barrel. The syringe also contains structure on the interior of the forward wall and the exterior of the piston for selectively attaching the piston to the forward wall so that the user can forcibly twist the piston to break the frangible means and draw the forward wall, including the hypodermic needle, into the syringe barrel. This design requires a compromise in the design of the syringe barrel. The barrel must be strong enough to remain intact during normal use yet weak enough to be sheared apart by any user regardless of strength.
Many prior art retractable needle syringes have deficiencies similar to that described above. In particular, the needle or the needle carrier of a retractable needle syringe must be securely held by the syringe barrel during normal use which often includes substantial hydraulic pressures experienced during injection especially with highly viscous liquids, and substantial forces including piercing rubber stoppers of medication vials. The syringe barrel must hold the needle carrier to a degree that it will not be overcome by the forces of normal use and still be disengagable through forces applied to a relatively flexible and weak plunger rod which extends from the open proximal end of the syringe barrel. Many prior art syringes recite designs that when made sufficient to withstand the forces of normal use, the needle or needle carrier cannot be easily disengaged. On the other hand, easy disengagement of the needle or needle carrier will lead to a structure that cannot withstand the forces of normal use.
Many prior art retractable needle syringes teach structures which require additional distal movement of the plunger to attach the plunger to the needle carrier or needle. Accordingly, after the injection process is completed, and the needle, is removed from the patient, the user advances the plunger rod further to secure the plunger rod to the needle carrier or needle for withdrawing into the syringe barrel. These structures leave valuable medication, or residual bodily fluid such as blood, in the syringe barrel. The volume of trapped or wasted medication is a function of the axial movement of the plunger from its position in the barrel at the end of injection to its further distal position in the barrel when it is secured to the needle or needle carrier. Also, this trapped medication or fluid is often forcibly expelled randomly into the healthcare environment while connecting the plunger to the needle carrier.
Many prior art syringe designs allow the repeated connection and disconnection of the plunger rod to the carrier and repeated withdrawal and re-exposure of the needle. Under certain circumstances this can be undesirable since it leads to accidental or unauthorized reuse of a contaminated syringe.
Also, most prior art retractable needle syringe designs do not provide structure for retaining the needle in its retracted position within the barrel, and although the needle may be permanently attached to the plunger rod preventing further reuse of the syringe, the needle may still be accidentally projected back out of the syringe where it presents a safety hazard.
Although the prior art teaches many different syringe assemblies having the capacity to withdraw the needle into the syringe barrel after use, there still exists a need for a simple, straight-forward, reliable, easily fabricated retractable needle syringe having locking structure for holding the needle in a position with respect to the barrel which is strong enough to resist the often substantial forces experienced during normal use and which is easily de-activated to lower the force required for withdrawing the needle into the barrel. There is also a need for a retractable needle syringe that does not waste medication by requiring additional axial motion of the plunger rod to secure the plunger rod to the needle carrier. There is also a need for a one-way locking mechanism so that when the plunger rod is secured to the carrier it will not disconnect from the carrier; finally, when the needle is withdrawn inside the barrel there is a need for the locking mechanism to prevent the needle from being later moved so that it projects out of the barrel.